The subject matter herein relates generally to the field of energy conservation, and, more particularly, to the field and related methods of heat transfer systems for warming water.
Buildings, such as hospitals, for example, may need to provide large quantities of hot water on demand to patients or processes. This can be a problem during peak usage hours, e.g., mornings when patients are generally taking warm showers or evenings when onsite laundries need hot water. Many hospitals use hot water storage tanks to store hot water so that patients and laundries may use hot water on demand without compromising the temperature. Water stored in hot water storage tanks are generally heated using fuel based heaters. Hot water temperature in storage tanks is recommended to be maintained at a temperature of 140° F. or higher in order to provide water at a desired temperature for use and/or to prevent the colonization of Legionella bacteria.
Energy costs may be high to maintain the hot water temperature. Another dilemma facing building engineers is the scalding of humans with the high temperature 140° F. storage tank water. One solution is anti scaling devices which deliver 120° F. temperature or less to the space. It is inefficient to heat water in storage tanks to 140° F. to prevent Legionella only to dilute to 120° F. at the point of service. Some building applications such as hospitals have eliminated the large storage tanks and use instantaneous heating of water to the delivery temperature of 120° F. using steam from large fuel burning boilers. With or without storage, emissions from the fossil fuel boilers or water heaters are known to cause increased pollution and global warming. Moreover, as fossil fuel becomes scarcer, the price of heating hot water will undoubtedly escalate.
Heat pump water heaters have been applied on a limited basis to heat water systems generally using an air source. These heat pump systems traditionally use synthetic refrigerants, such as R134a, which has a global warming potential approximately 1300 times greater than CO2. Therefore synthetic refrigerants such as R134a are known as “Greenhouse Gases” with a GWP of 1300 (Global Warming Potential). Even the latest synthetic refrigerants to replace R134a, such as proprietary refrigerant HFO1234yf (DuPont & Honeywell), reportedly has a GWP of 600 times that of CO2.
The aforementioned heat pump water heating systems are generally applied for space heating systems where the hot water is in a closed system. When potable water is needed to be applied to a heat pump, the choice is to apply an isolation water loop between the two systems to avoid cross contamination of the refrigerant and oil to the potable water. This custom engineered isolation loop is both costly and inefficient. Another solution for a heat pump water heater is to apply commercially available double wall vented brazed plate heat exchangers (BPHE). This approach has some disadvantages in that the heat exchangers are not cleanable or serviceable. Moreover the vented chamber between the two requires a great deal of costly heat transfer surface. Moreover these heat exchangers are vastly inefficient compared to single wall heat exchangers. Often the conventional heat pump water heating systems have vapor compression systems that are not dynamic enough to satisfy the demands of an instantaneous heating to 120° F. and therefore are applied with storage tanks maintaining approximately 140° F.
A natural refrigerant such as ammonia NH3 is more efficient than most synthetic refrigerants and has zero, global warming potential and zero ozone depletion potential as understood by those skilled in the art. However, ammonia cannot be applied to commercially available double wall BPHE's because there is copper in the system which is attacked by the refrigerant. Hydrocarbon refrigerants such as propane or isobutane have superior thermal characteristics to most synthetic refrigerants, leading to higher efficiencies and have excellent environmental performance with a GWP of 3. These refrigerants however are flammable in concentrations between approximately 2% to 9% of concentration and fail safe containment heat exchanger systems are not commercially available.
Another problem associated with known heat transfer systems, is that commercially available water to refrigerant evaporators are susceptible to freezing, particularly when the recovery temperature is below 48° F. or the system is operating in a low flow situation. The normal failure mode is ice forming on the water side of the refrigerant evaporators and the fragile wall breaches to the refrigerant side often destroying the compressor. In order to avoid this situation, prior art heat pump water heaters have thermostats to turn off the system if temperatures approach a predetermined low temperature. This however shuts off the production of hot water and compromises reliability. Chilled water heat recovery, as introduced by the prior art, is beneficial because heating and cooling are accomplished with the same kilowatt. However, the prior art fails to address building systems in cooler climates without an air conditioning load or even buildings in warm climates that do not have the availability to recover from a chilled water or condenser water system. Chillers with heat reclaim systems are focused on controlling leaving chilled water temperature while the recovered leaving hot water temperature is uncontrolled often falling below a set point during cool outside temperatures or low cooling load necessitating supplemental heating devices, such as fueled or electric resistance hot water heaters.
A need remains for a heat transfer system that overcomes these and other problems.